1. Field of the Invention
The present invention liquid crystal display device. In particular, the invention relates to a liquid crystal display device having a common signal line provided on a substrate.
2. Description of Related Art
In general, liquid crystal display devices are structured such that a pair of upper and lower electrode substrates having transparent electrodes are bonded to each other by use of a sealing member formed around an image display portion of the substrate and liquid crystal is filled therein. The liquid crystal display devices are classified into an active matrix type and a passive matrix type. An active matrix type liquid crystal display device as disclosed in Japanese Unexamined Patent Application Publication No. 11-149087 has a TFT array substrate where thin film transistors as switching elements are arranged in matrix (see Japanese Unexamined Patent Application Publication No. 11-149087). Then, the TFT array substrate is bonded to an opposing substrate through a sealing member. Liquid crystal is filled in between the TFT array substrate and the opposing substrate.
FIG. 13 is a plan view showing a corner portion of a peripheral portion of an image display portion of the TFT array substrate constituting the liquid crystal display device as disclosed in Japanese Unexamined Patent Application Publication No. 11-149087. In FIG. 13, reference numeral 1 denotes a wiring substrate having a TFT array; 2, a display area; 3, a scanning signal line; 5, a display signal line; 7, a common signal line; and 8, a sealing member.
In the liquid crystal display device, the sealing member 8 is formed around the display area 2. A pattern of the sealing member 8 has a portion overlapping with the scanning signal line 3 and a portion overlapping with the display signal line 5. Further, the sealing member 8 is arranged to overlap with the common signal line 7 at the corner of the display area 2. Assuming that the common signal line 7 is not provided, a base portion on which the sealing member 8 is to be formed becomes uneven: there is a step corresponding to the line height between a portion having the scanning signal line 3 or the display signal line 5 or the rest. Thus, a change in panel gap occurs due to the step at the corner of the image display portion, and causes an uneven display. However, in the technique as disclosed in Japanese Unexamined Patent Application Publication No. 11-149087, the common signal line 7 is formed near the corner of the wiring substrate 1 having the TFT array through the same process as that of either the scanning signal line 3 or the display signal line 5. As a result, a difference in level on the base portion underlying the sealing member 8 on the array substrate is eliminated or reduced. Therefore, the uneven display due to the change in panel gap can be controlled.
On the other hand, in the passive matrix type liquid crystal display device as disclosed in Japanese Unexamined Patent Application Publication No. 2003-186041, a transparent substrate having a segment electrode for applying voltage to liquid crystal is bonded to an opposing substrate having a common electrode through a sealing member. Liquid crystal is filled in between the substrate and the opposing substrate. A pixel is defined as a crossing point between the segment electrode and the common electrode.
FIG. 14 shows the structure of the liquid crystal display device as disclosed in FIG. 9 of Japanese Unexamined Patent Application Publication No. 2003-186041. FIG. 14 is a sectional view showing the structure of the liquid crystal display device. In FIG. 14, reference numeral 101 denotes a transparent substrate provided with a segment electrode (not shown) for applying a voltage to liquid crystal; 7a and 7b, a common signal line formed on the transparent substrate 101; 8, a sealing member formed on the transparent substrate; 19, a common electrode applied with a common signal; 102, a transparent substrate provided with the common electrode 19; 18, a conductive member; and 12, liquid crystal filled in a space defined by the above two substrates and the sealing member.
A driving circuit (not shown) or flexible substrate (not shown) formed on the transparent substrate 101 supplies a common signal to the common signal line 7. The common signal supplied to the common signal line 7 is applied to the common electrode 19 on the opposing substrate 102 through the conductive member 18. In this structure, the common signal line below the sealing member 8 has the two-layer structure, which enables a low line resistance. Further, the common signal line 7 is arranged to underlie the sealing member 8, so the sealing member 8 blocks infiltration of water from the outside of the panel to thereby improve a corrosion resistance of the common signal line 7.
however, the conventional liquid crystal display device has the following problems. That is, in the liquid crystal display device of FIG. 13, the common signal line 7 extends below the sealing member 8 pattern. Hence, it is necessary to minimize a difference in level on the base portion between a region where the sealing member 8 overlaps with the common signal line and a region where the sealing member 8 overlaps with the scanning signal line 3 or the display signal line 5. In this way, the abrupt change in panel gap that locally occurs can be suppressed. In general, it is rare that the scanning signal line and the display signal line largely differ in film thickness. Thus, in the liquid crystal display device as disclose in Japanese Unexamined Patent Application Publication No. 11-149087, the common signal line is formed using a material for the scanning signal line or a signal line to thereby level the base portion below the sealing member 8 pattern. That is, if the film thickness of the common signal line 7 increases, an abrupt change in panel gap locally occurs. Accordingly, with this structure, the common signal line should be formed using one of the scanning signal line and the material for a signal line. It is difficult to lower the resistance of the common signal line.
Further, there is a method of increasing a pattern width of the common signal line to lower the line resistance. However, if the line pattern width is increased, the width of a peripheral portion where the sealing member 8 is formed is increased. As a result, a frame area increases, resulting in a problem in that a compact liquid crystal display device with a small panel size cannot be realized.
In addition, in the liquid crystal display device of FIG. 14, the common signal line 7 underlying the sealing member 8 pattern has the two-layer structure, which realizes the low line resistance. However, if the common signal line of the two-layer structure is formed, there is a possibility that the height of the base portion largely changes at the border between a region where the display signal line or scanning signal line overlaps wit the sealing member 8 and a region where the common signal line overlaps with the sealing member. In this case, the uneven display is caused due to the abrupt change in panel gap at the portion concerned. Accordingly, in the structure of FIG. 14, a region where the two-layer line can be formed is limited for avoiding the abrupt change in panel gap. In some cases, this hinders the reduction in resistance necessary for high-speed driving.
As mentioned above, in the conventional liquid crystal display device, the uneven display is caused due to the abrupt change in panel gap, so the resistance of common signal line cannot be lowered. That is, if the common signal line has the two-layer structure, the panel gap abruptly changes. This leads to a problem in that the uneven display is caused, and display quality cannot be improved.
As discussed above, a conventional display device faces a problem in that, if the resistance of the common signal line is lowered, the uneven display is caused due to the abrupt change in panel gap, and the display quality cannot be improved.